Chemistry- A synthetic partial opiate agonist, pentazocine is commercially available as two separate salts. The hydrochloride salt, which is found in oral dosage forms, occurs as a white, crystalline powder. It is soluble in water and freely soluble in alcohol. The commercial injection is prepared from pentazocine base with the assistance of lactic acid. This allows the drug to be soluble in water. The pH of this product is adjusted to a range of 4-5. Pentazocine is a weak base with an approximate pKa of 9.0.
Storage/Stability/Compatibility - The tablet preparations should be stored at room temperature and in tight, light-resistant containers. The injectable product should be kept at room temperature; avoid freezing.
The following agents have been reported to be compatible when mixed with pentazocine lactate: atropine sulfate, benzquinamide HCl, butorphanol tartrate, chlorpromazine HCl, dimenhydrinate, diphenhydramine HCl, droperidol, fentanyl citrate, hydromorphone, hydroxyzine HCl, meperidine HCl, metoclopramide, morphine sulfate, perphenazine, prochlorperazine edisylate, promazine HCl, promethazine HCl, and scopolamine HBr. The following agents have been reported to be incompatible when mixed with pentazocine lactate: aminophylline, amobarbital sodium, flunixin meglumine, glycopyrrolate, pentobarbital sodium, phenobarbital sodium, secobarbital sodium, and sodium bicarbonate.
Pharmacology - While considered to be a partial opiate agonist, pentazocine exhibits many of the same characteristics as the true opiate agonists. It is reported to have an analgesic potency of approximately one-half that of morphine and five times that of meperidine. It is a very weak antagonist at the mu opioid receptor when compared to naloxone. It will not antagonize the respiratory depression caused by drugs like morphine, but may induce symptoms of withdrawal in human patients dependent to narcotic agents.
Besides its analgesic properties, pentazocine can cause respiratory depression, decreased GI motility, sedation, and it possesses antitussive effects. Pentazocine tends to have less sedative qualities in animals than other opiates and therefore is usually not used as a pre-operative medication.
In dogs, pentazocine has been demonstrated to cause a transient decrease in blood pressure. In man, pentazocine can cause increases in cardiac output, heart rate, and blood pressure.
Pharmacokinetics - Pentazocine is well absorbed following oral, IM, or SQ administration. Because of a high first-pass effect, only about 1/5th of an oral dose will enter the systemic circulation in patients with normal hepatic function.
After absorption, the drug is distributed widely into tissues. In the equine, it has been shown to be 80% bound to plasma proteins. Pentazocine will cross the placenta and neonatal serum levels have been measured at 60-65% of maternal levels at delivery. It is not clearly known if or how much pentazocine crosses into milk.
The drug is primarily metabolized in the liver with resultant excretion by the kidneys of the metabolites. In the horse, approximately 30% of a given dose is excreted as the glucuronide. Pentazocine and its metabolites have been detected in equine urine for up to 5 days following an injection. Apparently less than 15% of the drug is excreted by the kidneys in an unchanged form.
Plasma half-lives have been reported for various species: Humans = 2-3 hrs; Ponies = 97 mins.; Dogs = 22 mins.; Cats = 84 mins.; Swine = 49 mins. Volumes of distribution range from a high of 5.09 L/kg in ponies to 2.78 L/kg in cats. In horses, the onset of action has been reported to be 2-3 minutes following IV dosing with a peak effect at 5-10 minutes.
Uses/Indications - Pentazocine is labeled for the symptomatic relief of pain of colic in horses and for the amelioration of pain accompanying postoperative recovery from fractures, trauma, and spinal disorders in dogs. It has also been used as an analgesic in cats (see adverse effects below) and in swine.
Contraindications/Precautions - All opiates should be used with caution in patients with hypothyroidism, severe renal insufficiency, adrenocortical insufficiency (Addison’s), and in geriatric or severely debilitated patients.
Like other opiates, pentazocine must be used with extreme caution in patients with head trauma, increased CSF pressure or other CNS dysfunction (e.g., coma). Pentazocine should not be used in place of appropriate therapy (medical &/or surgical) for equine colic, but only as adjunctive treatment for pain.
Because reproductive studies have not been done in dogs, the manufacturer does not recommend its use in pregnant bitches, or bitches intended for breeding. Studies performed in laboratory animals have not demonstrated any indications of teratogenicity.
The drug is contraindicated in patients having known hypersensitivity to it.
Adverse Effects/Warnings - In dogs, the most predominant adverse reaction following parenteral administration is salivation. Other potential side effects at usual doses include fine tremors, emesis, and swelling at the injection site. At very high doses (6 mg/kg) dogs have been noted to develop ataxia, fine tremors, convulsions, and swelling at the injection site.
Horses may develop transient ataxia, and symptoms of CNS excitement. Pulse and respiratory rates may be mildly elevated.
The use of pentazocine in cats is controversial. Some clinicians claim that the drug causes dysphoric reactions in cats which precludes its use in this species, while others disagree and state that drug may be safely used.
Overdosage - There is little information regarding acute overdose situations with pentazocine. For oral ingestions, the gut should be emptied if indicated and safe to do so. Symptoms should be managed by supportive treatment (O2, pressor agents, IV fluids, mechanical ventilation) and respiratory depression can be treated with naloxone. Repeated doses of naloxone may be necessary.
Drug Interactions - When used with pentazocine, other CNS depressants (e.g., anesthetic agents, antihistamines, phenothiazines, barbiturates, tranquilizers, alcohol, etc.) may cause increased CNS or respiratory depression; dosage may need to be decreased.
a) 0.33 mg/kg slowly in jugular vein. In cases of severe pain, a second dose (0.33 mg/kg) be given IM 15 minutes later (Package Insert; Talwin®-V - Winthrop)
b) 0.33 - 0.66 mg/kg IV, IM or SQ (Jenkins 1987)
c) 0.4 - 0.8 mg/kg IV (Muir 1987)
d) 0.4 - 0.9 mg/kg IV (Thurmon and Benson 1987)
Note: Duration of analgesia may last only 10-30 minutes following an IV dose.
Monitoring Parameters -
1) Analgesic efficacy
2) Respiratory rate/depth
3) Appetite/bowel function
4) CNS effects
Client Information - Clients should report any significant changes in behavior, appetite, bowel or urinary function in their animals.
It is not approved for use in food producing animals (including horses to be used for food). All pentazocine products are Class-IV controlled substances.
Dosage Forms/Preparations/FDA Approval Status/Withholding Times -
Pentazocine Lactate Injection: 30 mg/ml (as base) in 10 ml vials; Talwin®-V (Pharmacia & Upjohn); (Rx) Pentazocine lactate injection is approved for use in horses and dogs.
Pentazocine Lactate Injection: 30 mg/ml in 10 ml vials and 1, 1.5, & 2 ml amps and pre-filled 2 ml carpuject syringes; Talwin ® (Sanofi Winthrop) (Rx)
Pentazocine HCl 50 mg & Naloxone HCl 0.5 mg Tablets (Scored); Talwin NX® (Sanofi Winthrop); Pentazocine & Naloxone HCl® (Royce) (Rx)
Pentazocine HCl 12.5 mg & Aspirin 325 mg Tablets; Talwin Compound Caplets® (Sanofi Winthrop) (Rx)
Pentazocine HCl 25 mg & Acetominophen 650 mg Tablets; Talacen Caplets® (Sanofi Winthrop) (Rx)
Note: All pentazocine products are Class-IV controlled substances and are prescription items only.
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